Truck drivers with sleep apnea who don’t regularly follow their CPAP treatment program are five-times more likely to crash, a new study finds.

The research included more than 1,600 US truck drivers with obstructive sleep apnea and a control group with an equal number of drivers without the condition. Those with sleep apnea were prescribed CPAP, a treatment that keeps the airways open during sleep.

The drivers with sleep apnea were given a CPAP machine that could be used both at home and in their truck’s sleeper compartment. The researchers tracked the truckers’ use of CPAP with an internal memory chip that was placed in each machine.

The rate of serious, preventable crashes was five times higher among truckers with sleep apnea who didn’t use their CPAP machine, compared to the control group. The crash rate among drivers with sleep apnea who always or sometimes used their CPAP machine was similar to that of the control group.